In a steel production factory which requires huge facilities, simplification of the process using so-called "on-line processing" has been investigated in order to save energy and to shorten the process. The "on-line processing" means to perform work such as rolling and heat treatment in one continuous manufacturing line. In the on-line processing, a method wherein a hot worked product is immediately quenched for utilizing its heat in the working is called "direct quenching". On the other hand, a method wherein the hot worked product is once cooled, and then subjected to heat treatment in a separate line is called "off-line processing", and quenching which is carried out in the off-line processing is called "reheating and quenching".
Recently, in the field of steel plate making by hot rolling, the off-line processing is diminished and most of the plates are produced in the on-line processing. In manufacturing of seamless steel pipes, however, the heat treatments such as quenching and tempering of the products is still mostly conducted in the off-line processing, because it is considered that quality and reliability of the product are more important. Needless to say, it is necessary for the off-line processing to include hardening facilities (a heating furnace and quenching equipment) and a tempering furnace in a separate line from the pipe manufacturing line.
In the conventional pipe manufacturing process, seamless steel pipes are produced in a consecutive hot working process comprising steps of piercing a steel billet by a pierce, extending and rolling by a plug mill or a mandrel mill, and shape-finishing by a sizer or a reducer. Sometimes, a press machine is used for piercing. Usually, the pipe manufactured in a working line forming is reheated, quenched and then tempered in a line separate from the pipe manufacturing line. In this way, the seamless steel pipes provided with sufficient properties such as strength, toughness and the sulfide stress cracking resistance are supplied to customers. The sulfide stress cracking is a crack which appears in high strength steels exposed to an environment containing sulfide, particularly hydrogen sulfide (H.sub.2 S). The "sulfide stress cracking" is referred to as "SSC" hereafter.
If the above mentioned usual quenching step is replaced by the direct quenching, simplification of the manufacturing facilities and reduction of the production are achieved. As mentioned above, "direct quenching" means the treatment, wherein the product, after hot working, is immediately quenched. In detail, it means a method to obtain a hardened metal structure, consisting of martensite or bainite by direct quenching from an austenite state at a temperature higher than Ar.sub.3 transformation point, in the hot working line.
For example, in Publication of Japanese Patent Application (referred to as PJPA hereafter) Nos. 58-224116, 60-75523 and 6-172859 disclose steel pipe manufacturing processes, including the direct quenching step such as enforced cooling, immediately after hot working. However, the pipes produced in the direct quenching process have coarse grain size in its microstructure and inferior toughness and corrosion resistance (SSC-resistance) in comparison with the pipes produced in the conventional off-line reheating and quenching process.
As mentioned above, the direct quenching tends to make the grain size of the product coarse in comparison with the conventional reheating and quenching. It had been thought that the direct quenching method was not suitable for manufacturing of a seamless steel pipe having high strength and high corrosion resistance because the pipe with coarse grain size is inferior in toughness and SSC-resistance, which are regarded as the most important properties of the seamless steel pipe.
As a method to refine crystal grains, a method has been proposed wherein the grain refining is performed by a combination of cooling and reheating for two phase transformations, i.e., transformation from austenite to ferrite and reverse transformation from ferrite to austenite. For example, a method wherein cooling and reheating steps are added intermediately to rough rolling and finish rolling is disclosed in PJPA No. 56-3626. Other methods wherein cooling and reheating steps are put together after finish rolling are disclosed in PJPA Nos. 58-91123, 58-104120, 63-11621 and 04-358023 respectively. Further, PJPA No. 58-117832 discloses a method in which two cooling and reheating steps are put into the process, one is during the rolling process, another is after rolling.
According to each method mentioned above, it is possible to refine the grains of the steel products which are directly quenched. However, each method includes the following problems.
1 The refinement of the grains is still insufficient for a requirement for higher level corrosion resistance.
2 Energy consumption to reheat the products, which have been once cooled to a temperature range for initiation and completion of transformation, to a temperature range wherein the reverse transformation is completed, is very large.
3 Since the above-mentioned methods require rather complicated and expensive facilities, the cost reduction of construction and operation is not so large in comparison with the off-line heat treatment.
In order to refine the grains further and improve hardenability of the steel, some methods wherein a steel product is direct quenched and tempered, after grain refining by hot working at non-recrystallization area and recrystallizing, are shown in PJPA No. 62-139815, No. 63-223125 and 64-55335.
In the method of the above mentioned No. 62-139815, strength and toughness of the product are improved by keeping the steel in a temperature range near to the finish rolling temperature, for recrystallization of austenite grains and retaining solute B (boron). This mechanism is based on a relationship between hardenability of the steel and the behavior of B during the process, from finish of hot rolling to quenching. The method of the said No. 63-223125 improves strength and toughness of the product by uniform fine grain structure of No. 8 or more of JIS grain size number. In this method, in order to get the said grain structure, the product is fully hot rolled in non-recrystallized temperature region, rapidly heated to a temperature for soaking for a short time, without cooling under Ar, transformation point, directly quenched, and tempered.
The above mentioned grain refining by direct quenching are concerned with a production technology for plate of low carbon steel in which recrystallization and grain growth occur relatively easily. If these methods are applied to a process of manufacturing a high strength corrosion resistant steel pipe for oil well use, it is difficult to obtain the same effect as the plate, since the seamless steel pipes for oil well use are made of medium carbon steels. Although rolling at a large working ratio is rather easy for the steel plate, especially of low carbon steel in the non-recrystallized condition in a comparatively low temperature area, the same rolling is extremely difficult for the steel pipe, especially of medium carbon steel, which is worked in a complicated rolling process. In other words, it is not easy to apply process steps for manufacturing steel plates to the process for producing steel pipes. In more detail, rolling at a large working ratio in the non-recrystallization temperature range below 1000.degree. C. in the general pipe rolling method, such as the plug mill method or the mandrel mill method, causes problems of over capacity of the mill or difficulty of drawing off the mandrel bar from the pipe after rolling. Accordingly, some countermeasures against these problems are necessary.
Inventions for recrystallization during or after rolling step in the direct quenching process for seamless steel pipe making are disclosed in PJPA Nos. 61-238917, 05-255749, 05-255750 and 05-271772.
The invention of the above mentioned No. 61-238917 is characterized by controlling the recrystallization ratio before quenching to more than 90% and using a steel of a specified chemical composition with defining heating condition after hot rolling precisely. However, No. 61-238917 states nothing about the rolling condition of the seamless steel pipe for reasons that an improvement of toughness, by changing rolling condition, is not practical. If the heat treatment disclosed in No. 61-238917 is simply applied to the general pipe rolling process, such as the plug mill or mandrel mill process, desirable uniform fine grain structure is not always obtained. Furthermore, the heat treatment probably will promote grain growth and generate coarse grains.
PJPA Nos. 5-255749 and 5-255750 disclose methods of direct quenching in which a hollow shell of specified chemical composition is forcibly cooled to 1100-900.degree. C. on the way of rolling and then rolled with a reduction of thickness in area of 15% or more into a pipe shell with an aimed outer diameter and thickness. Thereafter, the pipe is finish-rolled after re-heating at 900-1000.degree. C., and then directly quenched. Austenite grain size of the pipe finally produced in this method is 8.9 of ASTM grain size number at most because the grain has grown by the re-heating before finish-rolling, even if very fine grain structure is obtained during the hot rolling step. In addition, in the method described above, abnormal grain growth occurs frequently because of relatively low reduction of finish-rolling so that the pipe does not always have uniform fine grain structure. As mentioned above, the process comprising the re-heating step on the way of hot-rolling is not always favorable to make grains fine and uniform. The re-heating temperature can be in a range wherein the grain growth does not occur. In this case, however, the structure of the pipe becomes elongated grain or mixed grain structure because rolling after the re-heating was carried out in the non recrystallization temperature range. In particular, the elongated grain structure deteriorates hardenability of steel and increases anisotropy of the mechanical properties. Accordingly, it is difficult to use the seamless steel pipe produced in this method, as the steel pipe must have particularly good corrosion resistance.
PJPA No. 5-271772 discloses a method of manufacturing a steel pipe which has more than 90% martensite structure, wherein the pipe made of a billet of specified chemical composition by primary rolling is reheated to 900.about.1000.degree. C., and then is finish-rolled followed by direct quenching. However, No. 5-271772 does not state anything about the working conditions of steel pipe. As for this method, a uniform fine grain structure may not be always obtained, since the method is characterized by the re-heating of the pipe in this course of hot rolling is the same as the methods of preceding Nos. 5-255749 and 5-255750. Austenite grain size of the pipe obtained in this method, finally, is at most 7.3 of ASTM grain size number.
Methods for direct quenching of a steel pipe, the grains of which are refined, before quenching, by a combination of chemical composition of material and a specified arrangement of rolling mills, are disclosed in PJPA Nos. 5-271772, 6-172854, and 6-172858. In these methods, a hollow shell is formed into a finish product by two or more diagonally inclined roll mills (skew-roll mills) arranged in tandem. The deformation mode of rolling in the skew roll mill, contains a lot of shear strain component. In these methods, the hollow shell is rolled at a lower temperature than usual in each mill or in the first mill, and the temperature of the pipe is increased by working heat. The pipe is successively rolled in the skew-roll mill and finish rolled to the final products. Occasionally the pipe is re-heated before the finish rolling, i.e., after the last rolling in the skew-rolling mill. Under the rolling conditions of temperature and reduction ratio specified in these patent bulletins, the pipe receives severe deformation, even if the rolling is carried out in the skew-rolling mill, and the produced pipe has defects (surface defects) frequently. Furthermore, austenite grain size of the pipe, produced in this method, is 10.7 of ASTM grain size number at most because the reduction ratio in the finish rolling is too small.
Recently improvement of SSC-resistance of seamless steel pipes, particularly pipes for oil wells, has become an important subject, as the mining of high corrosive crude oil containing much sulfide has become active. As for the technology to improve SSC-resistance, methods for refining the grain structure of the pipe in a process comprising one or more reheating-quenching cycles were disclosed in PJPA Nos. 6-220536, 60-43424, 60-52520, 60-46318, 60-86208, 60-46317 and 60-86209, for example.
The above-mentioned No. 6-220536 discloses a method wherein a steel pipe, having a specified chemical composition, is reheated and quenched again after direct quenching. However, there is no description in No. 6-220536 about the working conditions of the steel pipe, especially the finish rolling condition just before direct quenching. If a steel pipe is subjected to direct-quenching after finish-rolling in the usual rolling method for seamless steel pipe by the plug mill or mandrel mill, the micro structure of the produced pipe does not always become ultra fine uniform grain structure because abnormal grain growth occurs frequently at reheating and quenching treatment, after direct-quenching. The pipe thus produced may be inferior in corrosion resistance.
The above-mentioned PJPA Nos. 60-43424 and 60-52520 disclose methods in which steels are reheated then quenched after direct quenching. In this method, the steels having specified chemical compositions are hot-rolled with not less than 20% reduction of thickness, at a temperature of 1000.degree. C. or less, just before direct quenching. Although these methods are characterized by finish rolling at lower temperature range, such as 1100.degree. C. or less, values of the reduction of thickness on rolling are about 40% at most, as indicated in examples. However, the steel only rolled with about 40% of reduction, never have satisfactorily refined austenite grains, after direct quenching, which become the initial grains in the reheating and quenching steps. Consequently, the reheating and quenching cycles should be repeated many times to obtain ultra fine grains of the steel.
PJPA Nos. 60-46318 and 60-86208 disclose a method of reheating and quenching of the pipes, wherein a steel having a specified chemical composition, is subjected to the primary-hot-rolling in austenite phase area and subjected to the secondary-hot-rolling, after being kept warm or being reheated, in order to suppress the initiation of transformation into ferrite phase and then the rolled steel is directly quenched. In this method, because the transformation is suppressed between the primary and secondary rolling steps, the austenite grains, after direct quenching, which become the initial grains in the reheating and quenching steps, can not be refined sufficiently. Therefore, the reheating and quenching cycles should be repeated many times, in order to obtain desirable fine grain structure. Since rolling conditions, especially the secondary rolling conditions before direct quenching, are not described at all in both of 60-46318 or 60-86208, it must be assumed that the secondary rolling (finish-rolling) is carried out under the usual conditions for general seamless steel pipe producing and then the pipe is direct quenched. In the steel pipe thus produced abnormal grain growth occurs frequently, contrary to expectation by the repeating of reheating-quenching cycle, therefore the steel pipe becomes inferior in corrosion resistance because the structure of the steel is not always ultra fine uniform grain.
PJPA Nos. 60-46317 and 60-86209 disclose methods of reheating and quenching of pipes, wherein a steel having specified chemical composition is primarily hot-rolled in the austenite phase area, and transformed into ferrite phase, thereafter reheated to austenite phase area once again, then secondarily hot-rolled and directly quenched. Austenite grains of the steel, after direct quenching, which will become initial grains in the reheating and quenching procedure become fine in this method because the steel transforms between the primary-hot-rolling and the secondary-hot-rolling. However, it is not preferable, in aspect of energy consumption increment, to cool the pipe to the temperature area of ferrite phase and then reheat to the area of austenite phase. Further, the method requires large equipment resulting in a remarkable rise of production costs. In addition, there is no description about rolling conditions, particularly the secondary hot-rolling condition, before direct quenching in both of 60-46317 and 60-86209. As mentioned above, when the secondary rolling (finish-rolling) is carried out under usual conditions for general seamless steel pipe making and the pipe is directly quenched, the abnormal grain growth occurs on the contrary in the reheating and quenching procedure, therefore the pipe becomes frequently inferior in corrosion resistance because the structure of the steel is not always ultra fine uniform grain.
A lot of studies have been done about relation between metallography of steel and SSC, in order to improve SSC-resistance of the steel. Some of the methods to improve SSC-resistance of the steel metallographically are as follows: 1 specifying chemical composition of the steel, 2 specifying metal structure, 3 improving heat-treatment technique, and 4 combination of the above-mentioned methods.
At first, as for the methods of specifying chemical composition, PJPA No. 62-253720 shows a method of specifying Si, Mn, P and Mo content and yield strength of the steel, No. 63-274717 shows a method of selecting high carbon steel, and No. 62-149813 and No. 63-238242 show methods of adding Zr to steel, respectively. Since W (tungsten) is an element of the same group in the periodic table and is similar to Mo in chemical properties, W has been added together with Mo as an alloying element. For example, PJPA No. 60-52520 discloses a method in which steel containing 0.05-0.80% of Mo+(1/2) W is directly quenched and tempered, in order to improve SSC-resistance by suppression of impurity segregation. However, all the methods described in these PJPAs are based on usual direct quenching, therefore, it is difficult to depress the SSC of the high strength steel which has been subjected to the conventional direct quenching method, even if the chemical composition of the steel is specified, as mentioned above.
As for the improvement of metal structure, it is known widely that the structure, which mainly consists of tempered martensite, is superior in SSC-resistance of the steel and it's fine grain structure is desirable. In addition, a method of forming bainite structure and a method of forming elongated grain structure are disclosed in PJPA Nos. 63-93822 and 62-30849, respectively. Further, as a heat treatment technique to obtain fine grain structure, other methods using rapid heating by an induction heating equipments etc. are disclosed in PJPA No. 54-117311 or 61-9519. These methods, however use the conventional reheating and quenching technology. Therefore, although their effect of improvement of SSC-resistance of the steel are recognized, the methods can not satisfy the industrial requirement for producing high quality seamless steel pipes at higher productivity, by the direct quenching technology using economical facilities.